1. Field of the Invention
The present invention relates to cationic resins and to their use in electrodeposition, and more particularly, relates to cationic resins containing blocked isocyanate groups and to their use in electrodeposition over steel substrates.
2. Brief Description of the Prior Art
Electrodeposition as a coating application method involves deposition of a film-forming composition under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because by comparison with non-electrophoretic coating means, electrodeposition offers higher paint utilization, outstanding corrosion protection and low environmental contamination. Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
To achieve the best corrosion resistance over steel substrates, the cationic electrodepositable composition is formulated with lead either as a pigment or as a soluble lead salt. However, lead is a very toxic material and as such presents many difficulties to the paint supplier and paint customer. The lead often finds its way into the effluent of the electrodeposition process which necessitates costly treatment processes to remove the lead. Also, to achieve optimum corrosion resistance, the steel substrate is usually pretreated prior to electrodeposition with a phosphate conversion coating and given a chromic acid rinse (chrome rinse) at the conclusion of the pretreatment process. Chromium is also a toxic material and effluent from the pretreatment process containing chromium must be treated and disposed of in a safe and ecological manner. This treatment process can be very costly.